Is there a good guide as to what the valid formats are here? I need to parse this information unless there is something handy function that I can use. I checked SPUtility and SPUtility.GetAccountName didn't seem to help.

I'm pretty sure that the "w" in "i:0#.w" means Windows and the "f" in "i:0#.f" means Forms, but I'm wondering if there is an enumerated list of possibilities somewhere.

Perfect. It doesn't quite explain all the pieces and parts, but it does provide code for parsing it. It will be interesting to see if it works with non-claims accounts (I suspect not).
–
Kirk LiemohnJun 8 '11 at 12:21

I see that there is a SPClaimProviderManager.IsClaimsUser() method which should help.
–
Kirk LiemohnJun 8 '11 at 14:11